Fuel oil or various liquid chemicals are stored in a tank. The tank may suffer some cracks due to time degradation. In this case, liquid in the tank leaks from the tank. It is very important to detect such leakage as soon as possible and cope with it adequately for preventing explosion and fire hazard, ambient pollution, or generation of poisonous gas.
As a detection device for detecting leakage of liquid in a tank in the shortest possible time, JP(A)-2003-185522 (Patent Document 1) has proposed a configuration that includes a measurement pipe into which liquid in a tank is introduced and a measurement slim-pipe provided below the measurement pipe and measures the liquid flow rate inside the measurement slim-pipe using a sensor section additionally provided to the measurement slim-pipe to detect a minute variation of the liquid surface in the tank, i.e., a liquid level variation.
In this liquid leakage detection device, an indirectly heated flowmeter is used as a sensor additionally provided to the measurement slim-pipe. In this flowmeter, a current is applied to heat a heating element, and a part of the heating value is allowed to be absorbed by liquid. Then, the heat absorption value of the liquid varies in accordance with the liquid flow rate. This characteristic is used to detect influence of the heat absorption based on a variation in an electrical characteristic value such as a resistance value caused by a temperature variation of a temperature-sensitive element.
The liquid leakage detection device disclosed in Patent Document 1 is substantially vertically inserted into the tank through a measurement port formed on the upper portion of the tank. The upper portion of the detection device is fixedly held by the tank measurement port, whereby the measurement pipe, measurement slim-pipe, and sensor section are fixed to the upper portion of the tank.
Patent Document 1: JP(A)-2003-185522